Electrostatic painting of various automobile parts, including doors and hoods, is commonly used today in the automotive industry. Electrostatic painting of sheet molding compound (SMC) substrates, for example, is desirable because it reduces paint waste and emissions as compared to non-electrostatic painting techniques. Electrostatic painting techniques require the substrate to be electrically conducting or to have an applied prep coat or primer, which is electrically conducting in order to display an increased paint transfer efficiency. Currently, an electrically conductive primer must be applied to a sheet molding compound composition article to be coated prior to electrostatically painting the article because, unlike steel, sheet molding composition is not conductive.
When using an electrically conducting primer, the path to ground is achieved via the conducting primer. An alternative technique is to use a grounding clip. This undesirably causes higher film builds near the grounding clip with film builds decreasing as the distance from the grounding clip increases. In addition, after several passes through the paint booth, significant resistance to ground may be encountered due to multiple paint layers on the buck itself.
As an alternative approach, electrically conductive thermoset composites have been produced for many years through the use of conductive grade carbon black pigments. However, this approach has included some complications.
The integral conductive network formed when using carbon blacks is not limited to the surface of the composite part alone. The entire matrix is rendered conductive, making it superior to conductive coatings in many applications. However, when formulating to achieve high levels of conductivity (for electrostatic painting, EMI, RFI) using carbon blacks, processing is drastically hindered because of the rheological impact on the SMC/BMC/RIM paste. Instances where these high levels are achieved and easily processed have encountered intermittent failures in conductivity due to instability in the conductive network of the carbon black pigments.
As a result, electrically conductive grade carbon black increases compound viscosity, modulus and conductivity. The tendency for these carbon blacks to flocculate (attractive forces acting to physically move carbon black particles together) provides carbon black with a low percolation threshold (the amount of a conductive material necessary to form a conductive network allowing for free election transfer between conductive particles) in most thermoset composite systems. Even with relatively low effective loadings, conductive carbon black pigments have a significant impact on the flow properties of thermoset composite systems. Therefore, glass reinforced thermoset composite production processes are presented with challenges when solely carbon black pigments are employed to provide conductivity.